1. Field of the Invention
This invention relates generally to plant fertilizers, and more specifically to means for enhancing the uptake of certain fertilizer components.
2. State of the Art
Nitrogen is one of the primary nutrients required by plants for robust growth. Most fertilizer compositions include nitrogen, most commonly in the form of urea. Urea applied to soil is enzymatically hydrolyzed to ammonia by an enzyme known as urease, which is found in soil microorganisms. However, this conversion is very rapid and exceeds the rate at which plants can utilize the released ammonia. The excess ammonia is either further converted to nitrites and nitrates by nitrifying bacteria in the soil (including Nitrosomonas and Nitrobacter species), which leaches into the water, or is lost to the atmosphere by denitrification, as known in the art. However, only about 40% to 50% of urea applied in conventional fertilizers appears to be utilized by plants, while the remainder is lost by the above-described processes. Obviously, a considerable amount of applied urea is wasted in that it is not utilized by plants.
Furthermore, the use of urea as the nitrogen source in fertilizers has serious environmental consequences. Nitrites and nitrates produced from ammonia by the nitrifying bacteria are not absorbed by the soil and leach into ground water and drinking supplies. Excess consumption of nitrites and/or nitrates causes a condition known as methemoglobinemia or "blue sickness", which is frequently fatal to infants less than 21 days old. Excess urea is often applied to crops to compensate for the inefficiency of urea uptake described previously. Much of the excess urea goes to run-off from the agricultural fields, resulting in acceleration of the eutrophication processes of surface water.
Despite these problems, urea remains one of the most inexpensive and widely available nitrogen sources for fertilizers. Therefore, attempts have been made to provide urea-containing fertilizers with reduced rates of nitrogen release to the environment.
One approach to the problem is to coat a urea-containing fertilizer pellet with a slow-release coating such as polyacrylamide or the like (e.g. European Patent No. 356,755 by Sommer et al, publ. Mar. 7, 1990). Polyacrylamide and similar coatings slow the rate of release of the urea into the surrounding soil. Another approach is to prepare a urea-formaldehyde conjugate composition. However, both of these approaches are relatively expensive, and thus the products are not practical for use by low-income farmers in Third World countries and elsewhere.
Another approach is to combine a urea-containing fertilizer with a urease inhibitor. The known inhibitors of urease in soil micro-organisms include diaminophosphinyl compounds and derivatives, various thio- and thio-phosphoryl compounds such as thiopyridine, thiopyrimidine and related compounds, and copper sulfate (see for example U.S. Pat. Nos. 4,932,991 issued Jun. 12, 1990 to Crenshaw and Radel; U.S. Pat. No. 4,824,783 issued Apr. 25, 1989 to Hendrickson et al; U.S. Pat. No. 4,670,038 issued Jun. 2, 1987 to Medina et al.; U.S. Pat. No. 3,523,018 issued Aug. 4, 1970 to Geissler et al, respectively). Many of the above compounds are also relatively expensive.
Accordingly, a need remains for an inexpensive, urea-containing fertilizer which has a reduced rate of conversion of urea to nitrites and nitrates. A need further remains for an effective urease-inhibiting composition having urease-inhibiting activity which can effectively reduce the amount of urea required to achieve a given yield of plant mass.